It has long been the objective of many inventions to produce electrical power inexpensively from solar electromagnetic radiation. Recently, there have been attempts to obtain very high energy conversion efficiencies by means of so-called multijunction photovoltaic converters, or by stacking solar cells with different bandgaps. Conversion efficiencies of the order of 32% for two junctions, 36% for three junctions and 38% for four junctions have been projected. Among the examples of such multijunction cells are epitaxially stacked monolithic AlGaAs/GaAs/Ge dual junction cells and monolithic CdZnS/InGaP/GaInAs/Ge 3-junction cells described in an article entitled "Design of High Efficiency Monolithic Stacked Multijunction Solar Cells" by L. M. Fraas and R. C. Knechtli, Proc. of the 13th IEEE Photovoltaic Spec. Conf., 1978. Such solar cells that are monolithically stacked on a single wafer are naturally cheaper, simpler and more easily cooled than a multiple wafer stack.
More recently, however, attempts have been made to stack upper and lower bandgap cells by attaching them mechanically rather than by growing them sequentially. The advantage of this method as disclosed in the Final Report dated May, 1980 by the instant assignee on Sandia Contract No. 07-6953A (pg. 34, et seq) resides in the flexibility with which the most convenient combination of materials may be selected for the cells so as to increase the chances of obtaining the high conversion efficiencies that are theoretically predicted. Such freedom of choice is also expected to contribute to the conservation of critical materials such as gallium.
Inherent in the stacking of solar cells to form an efficient photovoltaic converter, however, is the problem of making a void-free, transparent filler for thermal conduction as well as ohmic contacts with low series resistance. Use of glass as a support pad for a solar cell has been disclosed, for example, in U.S. Pat. No. 4,108,704 issued to William E. Horne, but the problem of internal electrical connection of the cells still remains.